The area became part of the Washington Territory when it was created later that year. Seattle was made the county seat on January 11, 1853. King, who had just been elected Vice President of the United States under President Franklin Pierce. The county was named after Alabamian William R. On Decemthe Oregon Territory legislature formed King County out of territory from within Thurston County. The local tribes provided the settlers with construction labor, domestic service, and help with subsistence activities. In the first winter after the Denny Party landed at Alki Point, the settlement at the point consisted of a few dozen settlers and over a thousand Native Americans. The Green River and White River were home for the Muckleshoot tribal groups. The Snoqualmie Indian Tribe occupied the area that would become eastern King County. Villages around the site that would become Seattle were primarily populated by the Duwamish people. When Europeans arrived in the region that would become King County, it was inhabited by several Coast Salish groups. (The others are Snohomish County to the north, and Pierce County to the south.) About two-thirds of King County's population lives in Seattle's suburbs. It is one of three Washington counties that are included in the Seattle– Tacoma– Bellevue metropolitan statistical area. King in 1852, in 1985 the county amended its designation and logo to honor Martin Luther King Jr., a prominent civil rights activist during the Civil Rights Movement. Originally named after US representative William R. The county seat is Seattle, also the state's most populous city.

The population was 2,269,675 in the 2020 census, making it the most populous county in Washington, and the 13th-most populous in the United States.

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